1. Field of the Invention
This invention pertains to electronic switching devices and, in particular, switching devices responsive to a signal which indicates that a circuit having a plurality of loads should be switched from series to parallel operation.
2. Description of the Prior Art
The device of this invention was designed specifically to solve a problem in electrically controlled fire extinguishing systems but as presented in this application it can be used with any system in which electromechanical or electrochemical devices must be actuated in response to a signal, such as an alarm signal, and in which safety requires that the system must work without failure. For purposes of discussion, a fire extinguishing system will be used as an example only.
In a fire extinguishing system the ultimate goal is to insure that the fire extinguishing agent gets out of its container and onto the fire in response to an alarm. Two devices are commonly used to activate extinguishers in response to an electrical signal. One of these devices is basically a solenoid which is slow in operation and requires high electrical energy for actuation. The other is a squib, which is basically a detonator which is fast to actuate and requires low electrical energy. The squib, like the solenoid, has an electrical coil which must be continuous. In an automatically controlled electrical alarm and extinguishing system, the continuity of the coils in the solenoids or squibs of each extinguishing unit is tested by a supervisory current. The squib, in particular, presents problems because it requires a very low current for actuation. Squibs must be absolutely uniform when connected in series and are, therefore, very expensive.
The device of this invention is particularly useful in systems using squibs. The squib is basically a blasting cap which is used to upset the pressure differential on a pressure differential extinguishing system valve. When a detection system generates an alarm signal which the control system processes into an extinguishing decision, an electrical potential is applied to the squibs to release the extinguishing agent into the hazard area to put out the fire. Squibs are generally connected in series such that the current which passes through one squib must pass through all the others in the circuit. Under the supervisory mode of operation this is ideal. A small supervisory current, which is not enough to detonate the squibs, is passed through each squib and returns to the control unit. The control unit looks for the presence of the supervisory current to determine whether or not the electrical continuity of the circuit has been maintained. If the supervisory current stops, the electrical continuity has been interrupted and a trouble signal is given. In the alarm mode of operation, however, when the squibs are connected in series, the current which passes through the first squib must also pass through all the other squibs in the circuit. This causes the voltage to be reduced when applied to each squib. If there are three squibs in a circuit, each one receives one-third the voltage, but they all receive the same current. Consequently, each squib must be carefully matched so that each squib detonates as a result of exactly the same amount of energy applied to the squib. If there is a deviation, the first squib to detonate would also be the last one to detonate. The precise matching of squibs has been a problem for many reasons. The nitro-cellulose gunpowder in squibs tends to deterioriate over time, but not at a continuous rate. The squibs must be continually replaced periodically to minimize this condition. Disposal of old squibs must be done in a safe manner. One solution to the squib problem is to wire them in parallel. However, this system has a flaw in that the circuit can no longer supervise the continuity of the coils or filaments in the squibs. A supervisory current would only indicate the presence or absence of wiring continuity to the end-of-line resistive device and returning to the control panel. In an alarm state, the polarity would be reversed and the current would flow through each squib. However, such a system depends on the electrical continuity of the squib. The filaments or coils of a squib are very sensitive to environmental factors such as shock, vibration the thermal cycling.
The series supervision/parallel actuation device of the present invention is an answer to this problem. It allows all the squib filaments to be supervised in series so that the control system knows their condition at all times and whether or not the wiring is intact. In the alarm state this device drives all the squibs in parallel. Each equib will receive all the current which it requires at full terminal voltage. If one squib is slower to react than the others, this factor is immaterial. This is an advance over the prior state of the art in which the system had to choose between getting all the squibs to detonate and not supervising each one (in which case it would not know which squib had failed) or supervising all the squibs but not being able to guarantee that all of them would detonate.
A similar problem exists for systems using solenoid type valves, which are typically wired in parallel. Using an end-of-line resistance device, the system has no way of knowing if each solenoid coil does in fact have electrical continuity. The system can only supervise for the electrical continuity of the wiring going out to the end-of-line device and back. Consequently, in prior extinguishing systems it has often been necessary to supply different and discrete circuits for each solenoid to be actuated. This increases the cost and complexity of the control equipment significantly because each circuit has to be independently fused, supervised and monitored. With the series supervision/parallel actuation device of this invention, a single circuit can be used to drive any number of solenoid actuated devices and the operator would be assured that should there be a break in the solenoid coils or the circuitry driving these coils, a supervisory indication of trouble would appear at the control panel.
Two prior U.S. patents are directed to other solutions of the problems discussed above. U.S. Pat. No. 3,952,809 discloses a series to parallel transfer circuit for initiator firing but requires one switch connected between each of the junctions between each pair of series connected activators and the power supply means. In the system of this patent, two wires for each switch have to be run from the control box to the switch for each suppressor initiator. This wiring is beyond that normally needed for the field loop. In the present invention, no additional wiring is required. U.S. Pat. No. 4,013,128 discloses a modular fire protection circuit in which some suppression units are activated in response to the activation of other suppression units. This system is also more complex than that of the present invention.